Excellence in Research: Carrier Dynamics and Charge Transport in Ultrawide Bandgap Semiconductors

Nontechnical description Ultrawide bandgap semiconductors can withstand higher voltages and higher temperatures than silicon and can operate at much higher frequencies. These materials are thus essential components of future high power, high speed electrical devices. Gallium oxide is a promising ultrawide bandgap material with a high breakdown voltage and a well-established low-cost production process. Current technology, however, does not support inclusion of effective positive charge carriers needed to make transistors and other devices from this material. This project explores methods to circumvent this difficulty by combining gallium oxide with other semiconductors and by use of alternative synthetic methods. Monitoring the evolution of charge carriers in these systems using advanced experimental tools enables a deeper understanding of the critical factors impacting the motion and energy of these charge carriers over time. These factors include interactions between charge carriers and a potentially imperfect atomic lattice, and how charge carriers travel between disparate materials. This knowledge can be extended beyond gallium oxide to other ultrawide bandgap semiconductors, many of which have similar physical properties. This project provides hands-on experience with advanced semiconductors to undergraduate students, contributing to the nation’s future workforce in a critical area. A wide group of students benefits through use of project materials and techniques in Physics courses. Finally, the project improves the research capacity of North Carolina Central University by supporting work in this emerging field. Technical description Gallium oxide is a highly promising ultrawide bandgap semiconductor, but an improved understanding of the fundamental physical mechanisms underlying carrier dynamics and charge transport in highly anisotropic material is required to optimize the performance of these candidate materials and to select new materials for testing. Devices based on β-Ga2O3 are also now limited by difficulty in p-type doping needed for efficient ambipolar device operation. This project focuses on characterizing the impact of crystalline orientation, dopant density and film morphology on carrier dynamics and charge transport in three potential routes to p-doped gallium oxide devices: (i) heterostructures of n-type gallium oxide and p-type gallium nitride produced by thermal oxidation and mechanical exfoliation; (ii) p-type gallium oxide nanowires and nanobelts fabricated through physical vapor deposition; and (iii) p-type polycrystalline Ga2O3 thin films produced by pulsed laser deposition. The current understanding of the evolution of excited charge carriers in these and other gallium oxide based systems is limited, due to strong electron-phonon interactions and highly anisotropic physical properties. These effects are further complicated by interfaces, local composition fluctuations, strain and grain boundaries that distort the crystal lattice and the energy landscape traversed by excited charge carriers in imperfect or doped systems, introducing new interaction pathways in the heterostructures and polycrystalline samples studied here. We use a multimodal approach including time resolved transient absorption, photoluminescence and terahertz spectroscopic studies to study these systems, enabling identification of critical factors limiting effective p-doping and charge transport. The potential outcomes of this research include an improved understanding of the impact of crystalline orientation, doping levels, local morphology and carrier densities on carrier dynamics and charge transport in gallium oxide nanostructures and thin films, as well as charge transfer in gallium oxide heterostructures. This project is funded by the U.S. National Science Foundation (NSF) Historically Black Colleges and Universities-Excellence in Research (HBCU-EIR) Program. HBCU-EIR supports research at public and private HBCUs to strengthen their research capacity and promote engagement with NSF. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.